Supplementary cementitious material and method for disposing of electric-arc furnace dust (eafd)

ABSTRACT

The present invention contemplates a cementitious additive that contains electric-arc furnace dust (EAFD) and a precursor as a replacement for up to about 30% of the cement in concrete. The additive includes about 50% by weight EAFD and about 50 wgt. % of precursor. The precursor includes about 55 wgt. % silica, about 10 to 20 wgt. % lime, about 2 to 5 wgt. % soda, about 5 to 15 wgt. % potash, about 2 to 6 wgt. % alumina and about 1 to 3 wgt. % magnesia. The method also contemplates the step of homogenizing the electric-arc furnace dust (EAFD) and precursor to produce engineered dust (ED) and then replacing up to 30% of the cement with a mixture referred to as ED i.e. homogenized EAFD and precursor.

FIELD OF THE INVENTION

This invention relates to a supplementary cementitious material andmethod for disposing of relatively large quantities of hazardous wasteand more particularly to a concrete composition and method for disposingof relatively large amounts of electric-arc furnace dust (EAFD).

BACKGROUND FOR THE INVENTION

Electric-arc furnace dust (EAFD) is a waste by product materialgenerated during steel making processes using EAF technology. 15-20 kgof the dust is generated per ton of steel product. The world generationof EAFD is estimated to be around 3.7 million tons per year. Further,with the continued increase in using EAF technology in worldwide steelproduction during the last decade, the problem of managing and handlingEAFD is also increasing.

EAFD is collected in bag houses and it comes in the form of very finepowder. The chemical composition of EAFD varies according to the qualityof steel scrap used in the manufacturing process. It is documented thatthe prevailing elements in the EAFD vary in concentration. However, themajor elements found in EAFD are oxides of zinc and iron.

Concentration ranges of elements in EAFD are as follows:

Element Range (%) Zn  2-46 Fe 10-45 Ca  1-25 Si 1-5 Al 0.1-1.5 Mg  1-12Na 0.5-1.8 K 0.35-2.30 Cd ?? Pb ??

Two forms of particles are observed in EAFD using scanning electronmicroscope (SEM). The predominant spherical forms observed in EAFD wereidentified to include a mix of metal oxides (Fe, Zn, Mn, OFe₂, O₃). Theminor angular-shaped particles were considered to be zincite (ZnO).

The x-ray diffraction (XRD) analysis of EAFD powder have shown thepresence of magnetite (Fe₂O₃), Franklinite (ZnFe₂O₄), lime and zincoxide (ZnO).

The mineralogical composition of EAFD shows that magnetite, lime,franklinite and zincite represent the major phases while magnezite,sylvite and hematite represent the minor ones.

One of the major problems encountered in the steel industry throughoutthe world is the management of the EAFDs which is listed as a hazardouswaste by regulations in most developed countries. This listing is to alarge extent due to the presence of leachable heavy metals. As such itcannot be disposed of directly as a landfill. Consequently, the problemsassociated with disposing of EAFD are sizable and costly.

Several routes for disposing of EAFD have been proposed in theliterature. They include solidification/stabilization (S/S)technologies, recovery of zinc and lead and recycling and utilization ofEAFD in cement and concrete manufacturing.

Even though S/S technologies that can be used to treat EAFD areimproving they are the least desirable approach to manage the EAFDdisposable because they offer no other benefits to the industry. Therecovery method of zinc and lead have been developed and successfullyapplied especially to the EAFD that contain high percentages of zinc andlead. In some cases, this approach is not economically feasible, butstill being used because it is the only viable option to manage thedust.

The concrete industry is one of the potential and energetic solutionsused to consume waste materials. Accordingly, many researchers haveinvestigated the incorporation of EAFD in concrete. It was observed thatthe concrete containing EAFD presented better mechanical behavior andimproved durability performance when compared to a reference concretemixture. However, the use of EAFD in concrete profoundly prolongs thesetting time of concrete for up to 30 hours at low replacement levels ofEAFD (less than 3%). This undesirable effect is considered as the maindrawback of wider utilization of EAFD in concrete. The retardation ofthe setting time is attributed to the presence of heavy metals such aszinc and lead as they are present in a soluble form in their carryingminerals.

The current invention is proposed to eliminate the problem with extendedsetting time associated with the use of EAFD. This limits its use inconcrete in a very small amount. This is achieved by bonding the heavymetals in EAFD in an insoluble crystalline phase with inheritedcementitious properties. As a result, the detrimental effect of zinc andother heavy metals on setting time is eliminated. Thus, higher amountsof EAFD can be utilized in concrete without negatively affecting thefresh properties.

There are two prior art patents awarded to inventors at the King SaudUniversity in Riyadh, Saudi Arabia. The first discloses the use ofelectric-arc furnace by product in concrete awarded to the followinginventors, Faisal H. Al-Sugair; Abdulaziz I. Al-Negheimish; and Rajeh Z.Al-Zaid. As disclosed in that U.S. Pat. No. 5,557,031, an improvedconcrete or mortar material wherein cement is replaced by electric-arcfurnace dust by amounts of from 0+% to 4% or more based on the amount ofcement by weight. In addition, the electric-arc furnace dust can be usedas an additive to obtain the same effect.

A more recent U.S. Pat. No. 8,152,917 of Abdulaziz I. Al-Negheimish;Rajeh Z. Al-Zaid and Abdulrahman M. Alhozaimy discloses a method formanufacturing concrete bodies or forms containing electric-arc furnacedust in the amount of up to about 4% at elevated temperatures to provideconcrete bodies. The concrete bodies have improved compressive strengthand slump retention capability. The method for producing the bodiesincludes the step of producing pre-selected amounts of electric-arcfurnace dust, water, cement and aggregate. The electric-arc furnace dustis mixed with water and then added to the cement and aggregate and mixedtogether at a temperature of about 32° C. and 45° C. to form a hotmixture. The hot mixture is added to a mold and caste at an elevatedtemperature to form a body with improved strength and slump retentioncharacteristics.

BRIEF SUMMARY OF THE INVENTION

In essence, the present invention contemplates a cementitious additivethat contains electric-arc furnace dust (EAFD) and a precursor as areplacement for up to 30 wgt. % of the cement in concrete. The additiveincludes about 50 wgt. % EAFD and about 50 wgt. % of a precursor. Theprecursor includes at least about 55 wgt. % silica, at least about 10 to20 wgt. % lime, about 2 to 5 wgt. % soda, about 5 to 15 wgt. % potash,about 2 to 6 wgt. % alumina and about 1 to 3 wgt. % magnesia.

In addition, a homogenuous mix of the EAFD and precursor are fed into akiln at about 1,000° C. and maintained therein at about that temperaturefor about 25 to 35 minutes for recrystallization to thereby form a newinsoluble phase of zinc and other heavy elements such as lead that arechemically bonded.

A second embodiment of the invention contemplates a method for replacingup to 30% of the cement in a concrete mix without adversely effectingthe setting time and the mechanical properties by adding the mixdescribed in the first embodiment of the invention to a concreteformulation that typically includes fine and coarse aggregate and water.

A third embodiment of the invention comprises or consists of 50 wgt. %of EAFD and a precursor in an equal amount wherein the precursorcomprises or consists of about 65 wgt. % silica, about 10 to 20 wgt. %lime, about 2 to 5 wgt. % soda, about 2 to 6 wgt. % alumina and about 1to 3 wgt. % magnesia. The EAFD and precursor are homogenized and heated(thermally treated) in a kiln preferably an electric kiln at about 1000°C. for about 25 to 35 minutes.

A fourth embodiment of the invention contemplates a method for disposingof electric-arc furnace dust (EAFD) comprising or consisting of thefollowing steps. In a first step a mixture of EAFD in an amount of about50 wgt. % and a precursor in an equal amount are provided. The precursorcomprises or consists of at least about 55-65 wgt. % silica, about 10 to20 wgt. % lime, about 2 to 5 wgt. % soda, about 5 to 15 wgt. % potash,about 2 to 6 wgt. % alumina and about 1 to 3 wgt. % magnesia.

In the fourth embodiment of the invention, the EAFD and the precursorare homogenized before feeding to the kiln by grinding one of the EAFDand precursor with the larger particle size to reduce the particle sizeto same particle size of the other so that each has about the sameparticle size.

In a further step, the homogenized EAFD and precursor are fed into anelectric or fueled kiln at about 1000° C. and processed therein for 25to 35 minutes to form an insoluble phase of zinc and lead which isreferred to as engineered dust (ED). Then about 21 wgt. % of theresulting mixture is mixed with about 49 wgt. % of cement and about 30wgt. % of coarse and fine aggregate and water to form concrete withoutadversely effecting the setting times.

A fifth embodiment of the invention relates to a concrete compositioncomprising and/or consisting of about 70 wgt. % of cement selected fromthe group consisting of Portland cement, ASTM Type I, Type II, Type Vand mixtures thereof; and about 30 wgt. % of thermally treated mix ofEAFD and a precursor (an engineered dust). The precursor consists of upto about 65 wgt. % silica, about 15 wgt. % lime, about 3.5 wgt. % soda,about 10.5 wgt. % potash, about 4 wgt. % alumina and about 2 wgt. %magnesia. In addition to the aforementioned base mix, about 70-74 wgt. %of coarse and fine aggregate and water in an amount of about 5 to 8 wgt.% are added.

The invention will now be described in connection with the followingfigures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphical representation illustrating the effect ofengineered dust (ED) replacement levels on the setting times of mortarmixes;

FIG. 2 is a graphical representation illustrating the compressivestrength development with the addition of ED at 3 and 7 days;

FIG. 3 is a x-ray defraction pattern of ED;

FIG. 4 is a scanning electron microstructural analysis of ED;

FIG. 5 is a graphical representation illustrating the effect of addingengineered dust (ED), micro silica (MS) and fly ash (FA) on the finalsetting times;

FIG. 6 is a graphical representation illustrating the compressivestrength of 5% ED, MS and FA with a water to binder (W/B) ratio of 0.3and sand to binder (S/B) ratio of 1.5;

FIG. 7 is a graphical representation illustrating the compressivestrength of 5% ED, MS and FA with W/B of 0.4 and S/B of 1.5;

FIG. 8 is a graphical representation illustrating the compressivestrength of 5% ED, MS and FA with W/B of 0.5 and S/B of 2;

FIG. 9 is a graphical representation illustrating the compressivestrength of 10% ED, MS and FA with W/B of 0.4 and S/B of 1.5; and,

FIG. 10 is a graphical representation illustrating the compressivestrength of 10% ED, MS and FA with W/B of 0.5 and S/B of 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The present invention is proposed to eliminate a problem associated withincluding electric arc furnace dust (EAFD) in concrete. The problem withthe use of EAFD in concrete is that it significantly prolongs thesetting time of the concrete for up to 30 hours at low replacementlevels of less than 3% EAFD. This undesirable effect is considered to bethe main drawback to the wider utilization of EAFD in concrete. Theretardation of the setting time is believed to be due to the presence ofheavy metals such as zinc and lead in a soluble form.

The current invention is proposed to eliminate the problem with extendedsetting time of concrete with EAFD therein and to significantly increasethe amounts of EAFD in concrete. This is achieved by bonding the heavymetals in EAFD in an insoluble crystalline phase with cementitiousproperties leading to what is referred to as engineered dust (ED). As aresult the detrimental effect of zinc and other heavy metals on settingtimes can be eliminated with the potential of added cementitiousproperties at the hardened state.

In the present invention, a chemo-thermal process transforms a designedmix comprising or consisting of 50% by wgt. EAFD and 50% by wgt. ofproprietary precursor into an alternative supplementary cementitiousmaterial. In the process the precursor has a lower melting point thanthat of the EAFD. The output of the process is a material withcementitious properties of zero effect on the setting time. The materialis referred to as engineered dust or ED. The ED replacement level canreach up to 30% without a substantial reduction in strength and yet withan overall setting time similar to that shown by a control mixture i.e.equal or less than six hours. It is important to remember that theaddition of 3% EAFD prolongs the setting time of concrete for up to 30hours.

FIG. 1 illustrates the effect of engineered dust on the setting time ofvarious mortar mixes containing from 0 to 30% as replacement levels forcement in the mix.

The effect of different ED replacement levels of 0.5, 10, 20 and 30% EDon setting times of mortar mixes with water to binder ratios W/B of 0.3and sand to binder ratios S/B of 1.5 are shown in FIG. 6. The binary mixcontaining up to 30% ED gives comparable setting times to the controlmix. The compressive strength of the mixes at 3 and 7 days is shown inFIG. 2. The binary mix containing 5% represents the optimum mix whichgives the highest strength. The binary mix with 10% ED gives comparableresults to that given by the control mix while the binary mixescontaining 20 and 30% ED gives lower strength than that given by thecontrol mix.

X-Ray Diffraction (XRD) and Scanning Electron Microscope (SEM) Analysis

The x-ray diffraction pattern shown in FIG. 3 represents that of thenewly formed crystalline phases.

The microstructural analysis of ED results in the formation ofnano-sized flaky (major) and rectangular (minor) structure as shown inFIG. 4 confirms the presence of the two main phases.

Final Setting Time

Different water binary mixes with W/B of 0.5 and S/B of 2 were preparedwith 5 and 10% ED, micro silica and fly ash. The final setting time forthese binary mixes are shown in FIG. 5. The mortar mix containing 5% EDhad a comparable final setting time to that given by a control mix andlower final setting time then that given by both mixes containing 5% MSand 5% FA. Furthermore, the mortar mix containing 10% ED gives lowerfinal setting time then that given by mixes containing 10% MS and 10% FAwhile still giving a setting time similar to that given by the controlmix. Accordingly, it can be concluded that the chemo-thermal process ofEAFD has overcome the drawbacks caused by Zn and other heavy metals andhas nullified the detrimental effect on setting time.

Compressive Strength

FIG. 1 illustrates the effect of engineered dust on the setting time ofvarious mortar mixes containing from 0 to 30% as replacement levels forcement in the mix. Those mixes are defined and identified in thefollowing table:

Composition of mortar mixtures Cement ED MS FA W/B S/B (%) (%) (%) (%)W3S1.5 0.3 1.5 100 0 0 0 W3S1.5ED5 0.3 1.5 95 5 0 0 W3S1.5MS5 0.3 1.5 950 5 0 W3S1.5FA5 0.3 1.5 95 0 0 5 W4S1.5C 0.4 1.5 100 0 0 0 W4S1.5ED5 0.41.5 95 5 0 0 W4S1.5MS5 0.4 1.5 95 0 5 0 W4S1.5FA5 0.4 1.5 95 0 0 5 W5S2C0.5 2 100 0 0 0 W5S2ED5 0.5 2 95 5 0 0 W5S2MS5 0.5 2 95 0 5 0 W5S2FA50.5 2 95 0 0 5 W4S1.5ED10 0.4 1.5 90 10 0 0 W4S1.5MS10 0.4 1.5 90 0 10 0W4S1.5FA10 0.4 1.5 90 0 0 10 W5S2ED10 0.5 2 90 10 0 0 W5S2MS10 0.5 2 900 10 0 W5S2FA10 0.5 2 90 0 0 10

A comparable study of different mortar binary mixes containing 0, 5 and10% of ED, micro silica (MS) and fly ash (FA) with water to binder (W/B)ratios of 0.3, 0.4 and 0.5 and sand to binder (S/B) ratios of 1.5 and 2were carried out as illustrated in FIG. 2. FIG. 6 shows that theaddition of 5% ED gives comparable results to that given by 5% MS and 5%FA and again still higher then that of the control mix at W/B ratio of0.3 and S/B of 1.5. At early and later stages 5% fly ash mix gives lowervalues then that given by either mixes. FIG. 7 shows that all mixes withW/B ratio of 0.4 and S/B of 1.5 and replacement levels of 5% havesimilar values at all ages. FIG. 8 shows similar trends i.e., mixes withW/B of 0.5, S/B of 2 and replacement level of 5% have similar values atall ages.

FIG. 9 shows the behavior of the mortar binary mixes with W/B ratio of0.4, S/B of 1.5 and replacement level of 10%. It shows that the control,10% ED and 10% FA give comparable results while 10% micro silica giveshigher values especially at later ages. FIG. 10 shows the behavior ofthe mortar binary mixes with W/B ration of 0.5, S/B of 2 and replacementlevel of 10%. This figure shows that the control and 10% ED givescomparable results while 10% FA and 10% micro silica give higher valuesespecially at later ages. This is attributed to the initial improvedapparent workability due to the presence of 10% ED. The insolublecrystalline flaky structure and ED improve the real logical propertiesof cement by decreasing the friction between the particles throughslippery action. Consequently, the incorporation of ED will reduce thewater demand and the chemical add mixture content; thus reducing theoverall cost of concrete production.

From the above, it is demonstrated that the presence of the engineeredEAFD (ED) brings the setting time back to its normal value. Withimproved cementitious properties enhanced it becomes a more valuablematerial.

The present invention will now be further illustrated by the followingexamples.

Example 1

A first embodiment of the invention relates to a cementitious additivecontaining electric-arc furnace dust (EAFD) and a precursor as areplacement for up to 30% of the cement in a concrete mix. Thecementitious additive comprises or consists of about 50 wgt. % EAFD andabout 50 wgt. % of a precursor and wherein the precursor contains atleast about 55 wgt. % silica, at least about 10 to 20 wgt. % lime, about2 to 5 wgt. % soda, about 5 to 15 wgt. % potash, about 2 to 6 wgt. %alumina and about 1 to 3 wgt. % magnesia and wherein a homogenized mixof the EAFD and precursor is fed into a kiln at about 1000° C. andmaintained therein for 25 to 35 minutes for re-crystallization tothereby form a new insoluble phase of zinc and other heavy elements thatare chemically bonded.

Example 2

In a second example of a cement additive in accordance with theinvention, the precursor comprises or consists of about 65 wgt. %silica, about 15 wgt. % lime, about 3.5 wgt. % soda, about 10.5 wgt. %potash, about 4 wgt. % alumina and about 2 wgt. % magnesia.

Example 3

A second embodiment of the invention relates to a method for replacingup to 30% of the cement in a concrete mix without adversely effectingthe setting time. The method comprises/consists of the following steps:reducing the particle size of the precursor to about the same particlesize of the EAFD; homogenizing about 50 wgt. % EAFD and about 50 wgt. %precursor wherein the precursor consists of silica, lime, soda, potash,alumina and magnesia, to produce a homogenized mixture; placing thehomogenized mixture in an electric kiln at about 1000° C. for about 25to 35 minutes for re-crystallization to form an insoluble phase of zincand heavy metals; and replacing up to 30% of the cement in a concretemix with said homogenized mixture after the heating of a mix of EAFD andprecursor in the kiln step.

Example 4

A further example of a method for replacing up to 30% of the cement in aconcrete mix without adversely effecting the setting time includes anengineered dust which is composed of a thermally treated mix of 50 wgt.% EAFD and 50 wgt. % precursor wherein the precursor contains about 65wgt. % silica, about 15 wgt. % lime, about 3.5 wgt. % soda, about 10.5wgt. % potash, about 4 wgt. % alumina and about 2 wgt. % magnesia.

Example 5

A still further example of a method for replacing up to 30% of thecement in a concrete mix without adversely effecting the setting timeincludes the step of homogenizing the EAFD and the precursor. The stepcomprises grinding one of the EAFD and the precursor with a largerparticle size to approximately the particle size of the other.

Example 6

A further example of a method for replacing up to 30% of the cement in aconcrete mixture without adversely effecting the setting time containsabout 70 to 74 wgt. % coarse and fine aggregates and about 5 to 8 wgt. %water and 8 to 17 wgt. % cement and 2 to 8 wgt. % ED (engineered dust)from a thermally treated mix of 50 wgt. % EAFD and 50 wgt. % precursor.

Example 7

A still further embodiment of the invention relates to a concretemixture consisting of Portland cement 70 wgt. %, about 30 wgt. % of EAFDand a precursor and wherein the precursor consists of about 65 wgt. %silica, about 15 wgt. % lime, about 3.5 wgt. % soda, about 10.5 wgt. %potash, about 4 wgt. % alumina and about 2 wgt. % magnesia and whereinEAFD and precursor are homogenized and heated in a kiln at about 1000°C. for about 30 minutes.

Example 8

An even further embodiment of the invention will be described by acementitious additive containing electric-arc furnace dust (EAFD) and aprecursor as a replacement for up to 30% of the cement in concretewithout adversely effecting the setting time of the concrete. Theadditive consists of about 50 wgt. % EAFD and about 50 wgt. % ofprecursor and wherein the precursor consists of 65 wgt. % silica, about15 wgt. % lime, about 3.5 wgt. % soda, about 10.5 wgt. % potash, about 4wgt. % alumina and about 2 wgt. % magnesia.

Example 9

The concrete mixture in which the cement is replaced with 30 wgt. % ED(engineered dust) which is formulated from a thermally treated mix of 50wgt. % EAFD and 50 wgt. % precursor where the precursor is homogenizedand consists of about 65 wgt. % silica, about 15 wgt. % lime, about 3.5wgt. % soda, about 10.5 wgt. % potash, about 4 wgt. % alumina and about2 wgt. % magnesia.

Example 10

The concrete mixture defined in Example 6 which includes about 70 wgt. %cement selected from the group consisting of ASTM Type I, Type II, TypeV and mixtures thereof.

Example 11

The concrete mixture defined in Example 9 in which the cement isPortland cement.

Example 12

A concrete mixture consisting of up to about 70 wgt. % Portland cement,and between about 5 and 30 wgt. % of ED where it is formulated from athermally treated mix of 50 wgt. % EAFD and 50 wgt. % precursor andwherein the precursor is selected from a group consisting of up to about65 wgt. % silica, about 15 wgt. % lime, about 3.5 wgt. % soda, about10.5 wgt. % potash, about 4 wgt. % alumina and about 2 wgt. % magnesiaplus coarse and fine aggregate and water.

Example 13

An additional embodiment of the invention includes a method fordisposing of electric-arc furnace dust (EAFD) comprising the steps offorming a mixture of about 50 wgt. % electric-arc furnace dust (EAFD)and 50 wgt. % of a precursor and wherein the precursor contains at leastabout 55 wgt. % silica, about 10 to 20 wgt. % lime, about 2 to 5 wgt. %soda, about 5 to 15 wgt. % potash, about 2 to 6 wgt. % alumina and about1 to 3 wgt. % magnesia; homogenizing said EAFD and the precursor bygrinding one of the EAFD and the precursor having larger particles thanthe other until the EAFD and the precursor have about equal sizeparticles; and feeding said homogenized EAFD and the precursor into akiln at about 1000° C. and rotating for about 30 minutes to form a newinsoluble phase of zinc and lead and adding about 5 wgt. % of theresulting mixture to about 16.5 wgt. % of cement and about 72 wgt. % ofcoarse and fine aggregate and 6.5 wgt. % water and mixing to formconcrete.

Example 14

The method for disposing of electric-arc furnace dust (EAFD) as definedin Example 13 in which the cement is selected from the group consistingof Portland cement, ASTM Type I, Type II, Type V and mixtures thereof.

Example 15

A method for disposing of electric-arc furnace dust (EAFD) consisting ofthe steps of forming a mixture of about 50 wgt. % electric-arc furnacedust (EAFD) and 50 wgt. % of a precursor and wherein the precursorcontains at least about 55 wgt. % silica, about 10 to 20 wgt. % lime,about 2 to 5 wgt. % soda, about 5 to 15 wgt. % potash, about 2 to 6 wgt.% alumina and about 1 to 3 wgt. % magnesia; homogenizing the EAFD andthe precursor by grinding one of the EAFD and the precursor havinglarger particles than the other until the EAFD and the precursor haveabout equal size particles; and feeding the homogenized EAFD and theprecursor into a kiln at about 1000° C. and rotating for about 30minutes to form a new insoluble phase of zinc and lead and adding about5 wgt. % of the resulting mixture to about 16.5 wgt. % of cement andabout 72 wgt. % of coarse and fine aggregates and 6.5 wgt. % water andmixing to form concrete without adversely effecting the setting time.

Example 16

A method for disposing of electric-arc furnace dust (EAFD) comprisingthe step of adding the material from Example 1 to a concrete mixturecontaining fine and coarse aggregate and water.

While the invention has been disclosed in connection with its preferredembodiments it should be recognized that changes and modifications maybe made therein without departing from the scope of the claims.

1. A cementitious additive containing electric-arc furnace dust (EAFD)and a precursor as a replacement for up to 30 wgt. % of the cement inconcrete, said cementitious additive comprising about 50 wgt. % EAFD andabout 50 wgt. % of precursor and wherein said precursor contains atleast about 55 wgt. % silica, at least about 10 to 20 wgt. % lime, about2 to 5 wgt. % soda, about 5 to 15 wgt. % potash, about 2 to 6 wgt. %alumina and about 1 to 3 wgt. % magnesia; and wherein a homogenized mixof said EAFD and precursor is fed into an electric kiln at about 1000°C. and maintained therein for 25 to 35 minutes for re-crystallization tothereby form a new insoluble phase of zinc and other heavy elements thatare chemically bonded.
 2. The cementitious additive according to claim 1in which said precursor consists of about 65 wgt. % silica, about 15wgt. % lime, about 3.5 wgt. % soda, about 10.5 wgt. % potash, about 4wgt. % alumina and about 2 wgt. % magnesia.
 3. The method for replacingup to 30% of the cement in a concrete mix without adversely effectingthe setting time, said method comprising the following steps: reducingthe particle size of the precursor to about the same particle size ofthe EAFD to thereby produce a homogenized mixture; wherein saidprecursor consists of silica, lime, soda, potash, alumina and magnesia;placing said homogenized mixture in an electric kiln at about 1000° C.for about 25 to 35 minutes for re-crystallization to form an insolublephase of zinc and heavy metals; and replacing up to 30% of the cement ina concrete mix with said homogenized mixture after the heating in thekiln step.
 4. The method for replacing up to 30% of the cement in aconcrete mix without adversely effecting the setting time, said methodconsisting of the following steps: reducing the particle size of theprecursor to about the same particle size of the EAFD to produce ahomogenized mix; wherein said precursor consists of silica, lime, soda,potash, alumina and magnesia; placing said homogenized mixture in anelectric kiln at about 1000° C. for about 25 to 35 minutes forre-crystallization to form an insoluble phase of zinc and heavy metals;and replacing up to 30% of the cement in a concrete mix with saidhomogenized mix after the heating in the kiln step.
 5. A method forreplacing up to 30% of cement in a concrete mix without adverselyeffecting the setting time according to claim 3, in which said precursorcontains about 65 wgt. % silica, about 15 wgt. % lime, about 3.5 wgt. %soda, about 10.5 wgt. % potash, about 4 wgt. % alumina and about 2 wgt.% magnesia.
 6. A method for replacing up to 30% of the cement in aconcrete mix without adversely effecting the setting time according toclaim 4, in which the step of homogenizing said EAFD and said precursorcomprises grinding one of said EAFD and said precursor with a largerparticle size to approximately the particle size of the other of theEAFD and precursor.
 7. A method for replacing up to 30% of the cement ina concrete mix without adversely effecting the setting time according toclaim 6, which contains 70 to 74 wgt. % aggregate and about 5 to 8 wgt.% water and 8 to 17 wgt. % Portland cement and 2 to 8 wgt. % ED. 8.(canceled)
 9. A cementitious additive containing electric-arc furnacedust (EAFD) and a precursor as a replacement for up to 30% of the cementin concrete without adversely affecting the setting time of theconcrete, said additive consisting of: about 50 wgt. % EAFD and about 50wgt. % of precursor and wherein said precursor consists of 65 wgt. %silica, about 15 wgt. % lime, about 3.5 wgt. % soda, about 10.5 wgt. %potash, about 4 wgt. % alumina and about 2 wgt. % magnesia. 10.(canceled)
 11. (canceled)
 12. (canceled)
 13. (canceled)
 14. (canceled)15. A method for disposing of electric-arc furnace dust (EAFD)comprising the steps of forming a mixture of about 50 wgt. %electric-arc furnace dust (EAFD) and 50 wgt. % of a precursor andwherein said precursor contains at least about 55 wgt. % silica, about10 to 20 wgt. % lime, about 2 to 5 wgt. % soda, about 5 to 15 wgt. %potash, about 2 to 6 wgt. % alumina and about 1 to 3 wgt. % magnesia;homogenizing said EAFD and said precursor by grinding one of said EAFDand said precursor having larger particles than the other until saidEAFD and said precursor have about equal size particles; and feedingsaid homogenized EAFD and said precursor into a kiln at about 1000° C.and rotating for about 30 minutes to form a new insoluble phase of zincand lead and adding about 5 wgt. % of the resulting mixture to about16.5 wgt. % of cement and about 72 wgt. % of coarse and fine aggregatesand 6.5 wgt. % water and mixing to form concrete without adverselyeffecting the setting time.
 16. A method for disposing of electric-arcfurnace dust (EAFD) according to claim 15, in which said cement isselected from the group consisting of Portland cement, ASTM Type I, TypeII, Type V and mixtures thereof.
 17. A method for disposing ofelectric-arc furnace dust (EAFD) consisting of the steps of forming amixture of about 50 wgt. % electric-arc furnace dust (EAFD) and 50 wgt.% of a precursor and wherein said precursor contains about 55 wgt. %silica, about 10 to 20 wgt. % lime, about 2 to 5 wgt. % soda, about 5 to15 wgt. % potash, about 2 to 6 wgt. % alumina and about 1 to 3 wgt. %magnesia; homogenizing said EAFD and said precursor by grinding one ofsaid EAFD and said precursor having larger particles than the otheruntil said EAFD and said precursor have about equal size particles; andfeeding said homogenized EAFD and said precursor into a kiln at about1000° C. and rotating for about 30 minutes to form a new insoluble phaseof zinc and lead and adding about 5 wgt. % of the resulting mixture toabout 16.5 wgt. % of cement and about 72 wgt. % of coarse and fineaggregates and 6.5 wgt. % water and mixing to form concrete withoutadversely effecting the setting time.
 18. A method for disposing ofelectric-arc furnace dust (EAFD) comprising the step of adding thematerial from claim 1 to a concrete mixture containing fine and coarseaggregate and water.
 19. (canceled)
 20. (canceled)